Natural Orbital Expansion of Interacting Geminals

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Abstract
An antisymmetrized product of “interacting” geminals is proposed for the form of a many‐electron wavefunction. The interacting geminals are given a natural orbital expansion where the natural orbitals of the geminals are also natural orbitals of the total wavefunction. This ansatz is more general than the strongly and weakly orthogonal geminal and identical geminal models and includes them as particular cases. However, the use of the interacting geminal wavefunction is not significantly more complicated for the purpose of calculations. Different constraints on the geminal occupation coefficients define the various geminal models and more than one model can be incorporated into a single wavefunction. The case of two singlet pairs of electrons is examined in terms of the interacting geminal model—density matrices, energy expressions, and variational equations being given.